The present invention relates to an antioxidant composition for lubricants, especially for synthetic ester lubricants, and to lubricants that include such an antioxidant composition. More particularly, this invention relates to a reaction product of substituted diphenylamines (DPA) and substituted N-phenyl-xcex1 (xcex2)-naphthylamine (PNA) and its use in lubricants. By careful selection of the mole ratio of DPA:PNA and reaction conditions, most of the DPA and PNA are converted to oligomeric products having improved properties over the starting materials.
Amine antioxidants are known, and have been widely used to improve the thermal-oxidative stability of synthetic ester lubricants used in the lubrication of moving parts operated at very high temperature, such as jet engines and hydraulic systems for military and commercial aircraft. In operation at high temperature in the presence of oxygen and catalytically active metals, the antioxidants are depleted. Oxidative oil degradation can create acidic by-products that degrade nearly metals and can form polymers which undesirably increase the viscosity of the lubricant. This oxidative degradation, in turn, can lead to oil insoluble sludge and deposits.
U.S. Pat. No. 3,655,559 discloses alkylated diphenylamines, U.S. Pat. No. 3,660,290 discloses alkylated N-aryl naphthylamines, and U.S. Pat. No. 3,804,762 discloses alkylated N-phenyl naphthylamines in combination with specific amino compounds which are useful as antioxidants for synthetic ester lubricants. U.S. Pat. No. 3,573,206 discloses reaction products from oxidation treatment of N-aryl naphthylamines and diarylamines to form homo-oligomers of PNA and cross oligomers of DPA and PNA and a high percentage of unreacted of DPA and PNA. These reactions are done in inert solvents, such as aromatic hydrocarbon or ketones, to avoid cross dehydrocondensation reactions described later.
Heretofore it has also been known that the treatment of various compounds with peroxide produces dehydrocondensation products having increased high temperature stability as antioxidants over the monomeric components. U.S. Pat. No. 3,492,233 discloses such a blend of a conventional polyester lubricating oil reacted in the presence of diaryl amines with certain organic peroxides to form dehydrocondensation products from the esters and diarylamines. These reactions require abstractable hydrogens on the polyester lubricants. U.S. Pat. No. 3,492,233 discloses a cross-dehydrocondensed product which consists of the stabilizer, such as a secondary aromatic amine or a hydroxyaromatic antioxidant, being chemically attached to the lubricating oil or other organic substances that have abstractable hydrogens under these reaction conditions. The product has increased high temperature stability over simple mixtures of the antioxidant in the oil.
U.S. Pat. No. 3,509,214 describes the high temperature air oxidation product or permanganate oxidation product from N-aryl naphthylamine or a combination thereof with diphenylamine. An article entitled xe2x80x9cFate of Amine Antioxidants During Thermal Oxidative Aging of Neopentylpolyol Ester Oilsxe2x80x9d in J. of Synthetic Lubrication 4:179-201 (1987) discloses that the high temperature air oxidation of diphenylamine yields as a product, a phenazine of the structure 
which is an insoluble sludge be avoided. As disclosed by Wieland in Chem. Ber. 39:1499-1506 (1906), the oxidation of diaryl amines with KMnO4 results primarily in a dimer of the diarylamines where the nitrogen atoms are bonded together.
It has now been discovered that an amine antioxidant composition having superior high temperature stability in lubricants such as ester fluids, but which has little dehydrocondensation product, can be made by reacting controlled amounts of organic peroxides with specific molar ratios of a diphenylamine to a N-phenyl-xcex1(xcex2)-naphthylamine, or their alkyl substituted derivatives. These reaction products have higher concentrations of oligomer than prior art disclosures in the presence of solvents with highly abstractable hydrogens. The composition is mainly homo-oligomers of DPA and cross oligomers of DPA and PNA. When added to an ester fluid, such as used for aviation lubricants, the inventive antioxidant provides excellent protection against oxidation of the lubricant.
Effective antioxidants for lubricants are reaction products of various substituted diphenylamines (DPA), substituted N-phenyl-naphthylamines (PNA), and organic peroxides. At mole ratios of DPA:PNA of about 1:1 to about 10:1, and at temperatures of about 70 to about 200xc2x0 C., primarily oligomeric products are formed from the amine molecules, which oligomeric products have enhanced performance as antioxidants over their precursors. The amount of peroxide varies from 0.5 to 3 moles per mole of total diaryl amines present. These reaction conditions were found to be critical to producing oligomers with degrees of polymerization in excess of 2, 3 or 4. Relatively low amounts of less active monomers and degradation products are associated with longer reaction times or more austere reaction conditions.
The foregoing reaction products are antioxidants and are suitable for use in lubricant compositions which comprise a major amount of an ester fluid lubricant and about 0.1 to about 10 weight percent, based on total weight of lubricant composition, of a reaction product as described above.